Sunny,
No, it actually has very little to do with the cost of food.
I grew up in the bar business, so I'm no prude, but we simply don't want to contribute to the prostitution problem in any way. Bad for business.
Also, we don't like the contrast between our regular guests and working girls hanging around our pool. Bad for business.
Also, the working girls in the DR are much more problematical than in, say, Canada. Bad for business.
Basically, admitting working girls to our resort is (aside from contributing to a problem we would like to see reduced) far more trouble than it's worth so we prefer that people looking for that sort of diversion do so elsewhere.
Heck, there are enough budget properties around that will rent you a room for a night for very little money that you could afford to take your girl there for the night and then go back to your ai afterwards for some peace and quiet.
(I hope this thread doesn't degenerate into another massive prostitution thread!!!)
Cheers!